The portion of the eye which forms the exterior surface thereof is known as the cornea. For the eye to provide normal vision, the cornea has a spherical shape, an aspherical shape being the cause of a visual defect which is known as astigmatism. Accordingly, providing an indication of the asphericity of the cornea corresponds to providing an indication of the extent of the astigmatism of the eye.
With ophthalmic surgery, for example, surgery involving cataracts and the implantation of intraocular lenses, ideally it is desirable to reconfigure the cornea after the operation to the same radius of curvature existing prior to the operation to thereby prevent or minimize astigmatism. Some operations require astigmatism to be induced at the end of surgery to allow for movement of the cornea during wound healing. In the human eye, the radius of curvature varies from individual to individual with a normal range between 40 and 50 diopters which corresponds to radii of curvature of approximately 8.44 millimeters to 6.75 millimeters, respectfully. In order to ascertain the radius of curvature of the cornea, measurements are taken along certain angles or meridians.
The measurement of the radius of curvature of a cornea may generally be utilized by an ophthalmologist to correct a patient's vision defects due to astigmatism. Additionally, the determination of the curvature of the cornea can be measured during eye surgery by an ophthalmic surgeon to establish the effects of the eye surgery upon corneal curvature at the actual time of surgery. Heretofore, such measurements have been taken using surgical instruments known as ophthalmometers or Keratometers.
Typically, a Keratometer uses a fixed illuminated object placed at a given distance from the anterior surface of the cornea. The distance is held constant by using the focus of a surgical microscope at high power utilizing minimum depth of field. This distance is the focal length of the objective lens of the microscope. The fixed illuminated object will form an image behind the anterior surface of the cornea which will vary depending on the curvature of the cornea. The surgical Keratometer has the ability to measure the size of this very small image accurately. Keratometers may provide quantitative as well as qualitative measurements of the curvature of the cornea in all meridians. A qualitative Keratometer provides the surgeon with an idea of the amount of astigmatism and the shape or gross shape of the cornea. A quantitative surgical Keratometer provides the surgeon with an actual measurement of the radius of curvature of the cornea or the dioptic power of the cornea.
Previously developed instruments for the measurement of the radius of curvature of the cornea include the instruments described in U.S. Pat. No. 4,046,463 issued to La Russa et al on Sept. 6, 1977 and entitled "Indicating an Asphericity of the Cornea of an Eye"; U.S. Pat. No. 4,157,859 issued to Terry on June 12, 1979 and entitled "Surgical Microscope System"; and U.S. Pat. No. 4,165,744 issued to Cravy et al on Aug. 28, 1979 and entitled "Dynamic Keratometry and Keratoscopy Method and Apparatus".
The La Russa et al instrument also known as the Troutman Keratometer, provides a qualitative measurement of the radius of curvature of the cornea. The Troutman Keratometer projects a circle of discontinuous dots upon the surface of the cornea. The degree of distortion of the dot pattern relative to a perfect circle is established by comparing the relationship between the dot pattern and a microscope eyepiece reticles. A flat cornea reflects a dot pattern closer to the outer reticle of the eyepiece, while a steep cornea reflects a dot pattern closer to the inner reticle. The maintenance of the Troutman Keratometer at a fixed distance from the cornea thus provides a measure of the degree of corneal curvature. Adjusting the cross hairs of the reticle of the microscope to coincide with the long axis of a reflected oval dot pattern provides reference points for establishing the approximate amount of astigmatism or meridional error present. Since image focus and measurement of the degree of corneal curvature require that the Troutman Keratometer be fixed, the system is incapable of providing dynamic keratometry. That is, it is not possible to dynamically detect meridional error of all positions of the meridian lying between the cornea center and corneal periphery. The focused dot pattern crosses each corneal meridian at only one point.
The Terry surgical microscope system projects onto the surface of a cornea a circular image and utilizes a prism for optically splitting the image so projected into a plurality of substantially identical images viewable through one or both binocular eyepieces of a microscope. By using the zoom mechanism of the microscope or by utilizing rotary prisms, the system positions the images in a predetermined alignment for indicating the curvature and configuration of the cornea. The projected image may be optically split by using prisms, lenses or mirrors insertable into the optical path within the microscope. Where a fixed location prism array is utilized for optically splitting the image, the optical system of Terry is adjusted by varying the magnification powers of the microscope to bring the projected image and split images into a predetermined alignment and configuration. Where rotating prisms are utilized in a fixed position within the optical path for optically splitting the image, the optical system of Terry is adjusted by varying the prism power to bring the images into a predetermined alignment and configuration. The Terry system is a quantitative Keratometer in which the radius of curvature of the cornea is calculated.
The Cravy instrument is also a qualitative device for determining corneal curvature. The Cravy instrument projects a continuous circle of light onto the surface of a cornea from a housing for viewing the reflected image. The housing is moved toward the cornea so that the image is reflected from more peripheral portions of the cornea, such that the diameter of the reflected image progressively increases toward the cornea periphery to detect meridional errors through the angular length of the meridians of the image.
Such previously developed surgical instruments for measuring the radius of curvature of a cornea have been relatively complex in structure, difficult to manufacture and expensive. Additionally, qualitative Keratometers do not provide the ophthalmic surgeon with sufficient information to correct for corneal defects. The use of previously existing quantitative surgical Keratometers such as the Terry Keratometer depends upon the zoom feature of a microscope which must be used with the Keratometer and further requires modification of the microscope. Therefore, such a Keratometer is dependent upon the operation and the components of the associated microscope. An important quality of a Keratometer must be its accuracy. The accuracy of a surgical Keratometer is two fold, one being the absolute accuracy and the other being a relative accuracy. The absolute accuracy of the Keratometer is the value of the reading of a meridian compared to the actual value of the specific meridian and depends on the accuracy of focusing of a microscope. If the microscope is not absolutely focused, then the Keratometer cannot read an absolute value which presents a problem for fitting contact lenses. The important measurement for astigmatism control is the difference in value of the two major meridians, usually vertical and horizontal. The difference of the two values determines the astigmatism and can be controlled by closure techniques for closing the cornea at the end of corneal surgery. This accuracy reflects the relative accuracy which, because the measurements are taken at the same focal length, tends to result in relatively high accuracy. Due to these requirements, such surgical instruments require a high degree of accuracy. Additionally, since such surgical instruments are utilized during surgery, they must be easy to operate and provide immediate information to the ophthalmic surgeon.
A need has thus developed for a surgical instrument for measuring the radius of curvature of a cornea that does not depend on the optics of a microscope thereby eliminating the need for zoom microscopes. A need has further arisen for a surgical instrument that can be mounted on any existing microscope without interferring with the operation of the microscope during surgery or during normal measurement procedures. A need has further arisen for a surgical instrument for measuring the radius of curvature of a cornea which is accurate, both absolutely and relatively, reliable and inexpensive resulting in a maintenance free instrument.